1. Field of the Invention
The present invention relates to novel, unblocked polyisocyanate mixtures suitable as crosslinking agents for powder coatings, to a process for their production and to their use for the production of polyurethanes, in particular as hardeners for polyurethane powder coatings.
2. Description of the Invention
Combinations of organic polyhydroxyl compounds and blocked polyisocyanates are valuable binders for heat-crosslinkable powder coatings. See, for example, DE-OS 2,105, 777, DE-OS 2,542,191, DE-OS 3,143,060, DE-OS 2,735,497, DE-OS 2,842,641, EP-A-0,286,799, EP-A-0,218,040, EP-A-0,460,963, DE-OS 2,801,126, EP-A-0,403,779, WO 91/155320 U.S. Pat. Nos. 3,857,818, 4,375,539, EP-A-0,409,745, DE-OS 2,812,252, DE-OS 2,946,085 and DE-OS 3,434,881.
Common to these prior publications is the disadvantage that, on 2O thermal crosslinking, the compounds used as blocking agents are released and escape to the environment. For ecological and occupational hygiene reasons, therefore, particular precautions have to be taken when they are processed to purify the exhaust air and/or recover the blocking agent.
An attempt to overcome this fundamental disadvantage is the use of IPDI powder coating hardeners which contain linear uretdione and urethane groups but do not contain blocking agents. These hardeners have terminal urethane, urea or free isocyanate groups (EP-A-0,045,994, EP-A-0,045,996 and EP-A-0,045,998). Crosslinking proceeds by thermal cleavage of the uretdione groups. The disadvantage with these crosslinking agents is their strictly linear nature, which is required for the synthesis process and which does not permit the lacquer to be branched to achieve good solvent and scratch resistance, and increased hardness by varying the hardener.
EP-A-0,193,828, EP-A-0,224,165 and EP-A-0,254,152 describe certain polyisocyanates which are solid at room, temperature and contain isocyanurate and/or urethane groups and have free isocyanate groups bonded to tertiary (cyclo)aliphatic carbon atoms as the crosslinking component for PUR powder coatings. Because of the inertness of the tertiary NCO groups, these polyisocyanates may be mixed in unblocked form with OH-functional powder coating binders at temperatures above their melting point without any undesirable premature reaction occurring. In this manner, it becomes possible to produce PUR powder coatings which are stable in storage and contain no blocking agents and which crosslink at 150.degree. to 220.degree. C., preferably 170.degree. to 190.degree. C. into highly crosslinked, glossy lacquer films with good hardness, elasticity and solvent resistance.
Within the context of efforts to reduce still further solvent emissions from coating compositions, the use of powder coatings is now also being discussed for a range of applications which have previously been reserved for conventional solvent-based or water-based lacquer systems, for example, for original equipment automotive lacquer coatings. To be useful for this application, the powder coating systems must be curable at temperatures below 150.degree. C., in particular, below 140.degree. C., without eliminating blocking agents. This requirement:cannot be satisfied by the above-mentioned polyisocyanate crosslinking agents having free tertiary bonded isocyanate groups and which do not contain blocking agent.
An object of the present invention is to provide novel, unblocked polyisocyanates which are suitable as hardeners for powder coating compositions which may be crosslinked at baking temperatures below 150.degree. C. and which provide solvent and chemical resistant coatings having good optical and mechanical properties.
This object may be achieved with the polyisocyanate mixtures according to the invention which are described below in greater detail along with the process for their production. The polyisocyanate mixtures according to the invention are based on the surprising observation that incorporating ester and/or carbonate groups into polyisocyanates with isocyanate groups bonded on tertiary carbon atoms permits the reactivity of these isocyanate groups to be raised to such an extent that it is possible to produce powder coatings which do not contain blocking agents and which crosslink at baking temperatures well below 150.degree. C., for example, at 120.degree. to 140.degree. C.